The present invention relates to an optical axis compensator used in a survey instrument or a measuring instrument for forming a horizontal or vertical reference plane or a reference plane tilted at a predetermined angle using laser beam.
In a survey instrument or a measuring instrument for forming a reference plane, an optical axis compensator has been used in the past for automatic compensation of optical axis, i.e. an optical axis compensator of liquid surface reflection type based on an absolute horizontal plane formed by liquid or an optical axis automatic compensator of liquid transmission type based on wedge-like change which occurs when liquid is tilted.
Description will be given below on a conventional type optical axis compensator of liquid surface reflection type referring to FIG. 14.
In the figure, reference numeral 1 represents a liquid sealing container provided in a main unit of the device, such as a measuring instrument, and a free liquid surface 2 is formed by a transparent liquid sealed in the liquid sealing container 1. Into the liquid sealing container 1, a light beam emitted from a light source 3 is projected to the free liquid surface 2 at a predetermined angle via a collimator lens 4, and the projected light beam is totally reflected by the free liquid surface 2. The light beam reflected by the free liquid surface 2 is then reflected in a vertical direction by a reflection mirror 11.
The free liquid surface 2 forms a horizontal plane regardless of tilting of the measuring instrument main unit. When the measuring instrument main unit is tilted, i.e. when the liquid sealing container 1 is tilted, an incident angle of the light beam entering the free liquid surface 2 is changed depending on each case. This change of the incident angle is uniquely determined by tilting of the liquid sealing container 1, and a reflection angle on the free liquid surface 2 is uniquely determined by the incident angle. Therefore, by arranging an anamorphic optical system 7 comprising prisms 5 and 6 and a beam expander 10 comprising convex lenses 8 and 9 along the optical axis of the reflected light beam, an optical axis of the reflected light beam is compensated, and a reflection optical axis always directed in a vertical direction can be obtained.
Using the vertical reflected light, a vertical reference line and a horizontal reference line can be formed, or a horizontal reference plane can be formed by rotating the horizontal reference line.
In the optical axis compensator as described above, automatic compensation of the optical axis depends upon tilting of the liquid surface and the optical system. For this reason, the range of compensation is very narrow, and the angular compensation obtained is about 10 minutes at the most. As a result, the tilting of the measuring instrument main unit must be manually compensated in advance up to a range that can be automatically compensated. Further, from the unexpected reasons, if it is out of the range of automatic compensation, it must be corrected manually each time, and this leads to troublesome procedure and poor working efficiency.